1. Field of the Invention
The present invention relates to Francis pump-turbines and, in particular, to a Francis pump-turbine capable of being operated at high-efficiency when the turbine is in a partial load operation.
2. Related Art
General Francis pump-turbines can freely select either a generating mode or a pump mode by switching the rotation of a motor generator from normal rotation to reverse rotation or vice versa.
FIG. 11 illustrates a general structure of a Francis pump-turbine, and as shown in FIG. 11, when the turbine generator is in operation, the Francis pump-turbine with this structure guides driving (operating) water, which is supplied from a high-level reservoir and a hydraulic pipe, both are not shown, to a spiral casing 1, to a runner 4 through a stay vane 2 and a guide vane 3 to rotate the runner 4, and supplies the power (running torque) generated at that time to a motor generator, not shown, via a main shaft (rotation shaft) 5.
While the pump is in operation, the Francis pump-turbine rotates the runner 4 in a direction reverse to that at the turbine generation by a driving force of the motor generator to guide the driving water from a draft tube 6 communicating with a lower reservoir to a runner chamber 7, where the driving water is given with energy from the runner 4 and pumped to the upper reservoir through the guide vane 3, the stay vane 2, and the casing 1.
The guide vane 3 opens or closes the vane according to fluctuations in electric power demand to control the flow rate of the driving water supplied from the casing 1 during the turbine generation.
The runner 4 has runner blades 8 mounted around the circumferential direction of the main shaft 5 at regular intervals, in which both ends in a height direction of the runner blade 8 is supported by a crown 9 and a band 10 to provide a flow channel between the runner blades 8.
FIG. 12 is a meridian view of the runner 4, extracted from part “A” of FIG. 11, where the meridian view is a developed view including the main shaft 5.
The trailing edge 11 of the runner blade 8, having both ends supported by the crown 9 and the band 10, has generally been constructed such that the radial position of the crown-side end 12 is set at about 50 percent of the radial position of the band-side end 13 from the main shaft 5 of the trailing edge 11.
In other words, the curved portion connecting the crown-side end 12 defined by the trailing edge 11 of the runner blade 8 and the crown 9 and the band-side end 13 defined by the trailing edge 11 and the band 10 bulges or expands out smoothly to the main shaft 5, wherein the relationship between a distance Rc from the main shaft 5 to the crown-side end 12 and a distance Rb from the main shaft 5 to the band-side end 13 has substantially been set to Rc=0.5 Rb.
As shown in FIG. 13, the runner 4 viewed from the outlet of the runner blade 8 (adjacent to the draft tube 6) has a structure in which the crown-side end 12 and the band-side end 13 of the trailing edge 11 of the runner blade 8 are connected with a straight line.
The crown-side ends 12 and the band-side ends 13 of all the trailing edges 11 have the same circumferential angle in the polar coordinates indicated by a radial distance from the main shaft 5 and a circumferential angle.
The runner 4 having the runner blades 8 of such a shape decreases significantly in runner efficiency if the flow rate goes out of a design point. For example, when the flow rate of the water falls below a design point, the driving water is affected by a centrifugal force to drift toward the band 10. When the flow rate of the water exceeds a design point, the water drifts toward the crown 9, causing a loss due to so-called secondary flow.
In order to reduce the loss due to the secondary flow to improve the partial load efficiency of the turbine, it has been proposed that the curved portion connecting the crown-side end 12 and the band-side end 13 of the trailing edge 11 of the runner blade 8 is changed from the position indicated by the broken line in a conventional structure to the position adjacent to the main shaft 5, indicated by the solid line, with which the flow line ST1 indicated by the broken line is shifted to the flow line ST2 indicated by the solid line to make the flow uniform. This method, however, has induced backflow to the runner blades 8 during the operation of the pump, extremely decreasing pump operation efficiency.
This leads to a need for tradeoff in designing hydraulic machinery in which not only the partial load efficiency of the turbine but also pump operation efficiency are improved.
A technique for improving the runner efficiency when the pump is in operation is disclosed in JP-A-2000-136766 (Patent Document 1), in which the leading edge of the runner blade is in form of a curve recessed in the rotating direction during the operation of the pump.
The Patent Document 1 discloses a pump-turbine runner having blades whose portions of leading edges (that is, trailing edges when the turbine is in operation) are in the form of curves recessed in the rotating direction during the operation of the pump. However, it provides no solution to the drift of the driving water when the turbine is in operation.
On the other hand, the technique proposed in FIG. 14 does not solve the problem of backflow induced when the pump is in operation even if the position of the trailing edge 11 indicated by the broken line in the conventional structure is shifted downstream to the position indicated by the solid line to make the flow line ST2 uniform, thereby improving the partial load efficiency of the turbine. Therefore, the tradeoff is needed as well as in the Patent Document 1.